Hybrid ablative thermal protection systems and associated methods

ABSTRACT

A method of forming a thermal protection system for high speed aircraft is described. The method includes mechanically working an uncured ablator material into a first surface of a felt layer such that the ablator material penetrates a distance into a thickness of the felt layer thereby forming an region that has a mixture of felt and ablator material, adding additional uncured ablator material to the worked ablator material, and curing the combined ablator material.

BACKGROUND

The field of the invention relates generally to ablative materials, andmore specifically to a hybrid ablative thermal protection system.

Ablative materials have been used in a variety of applications toprotect and insulate structures that are subjected to extreme thermalconditions. For example, many aerospace vehicles that traverse, exit,and enter the atmosphere of the Earth travel at high velocities, and asa result, their exterior aerosurfaces, and to some degree theirsubstructure, experience high aerothermal loads. Aerothermal loads havebeen managed using a variety of techniques including insulation, radiantcooling, active cooling, conduction and convective cooling, and by phasechange or ablative materials. Generally, ablative materials are appliedto the affected aerosurfaces to absorb the extreme heat in order toinsulate the vehicle from the thermal environment.

The thermal management technique of ablation has been widely used for avariety of applications since the early 1930s. Ablative materials wereused in early rocket systems for nose cap protection and have also beenused as re-entry heat shields on the Gemini and Apollo space vehicles,and further on many modern rocket nozzles. Many of these materials,although suitable for use in the aforementioned applications, havehandling and longevity issues that preclude application on a system thatis subjected to frequent handling and that may be stored for severalyears prior to use.

Known ablative materials comprise a variety of constituent components,each at certain percentages by weight or volume, to achieve the desiredlevel of thermal protection and other physical properties. Generally,ablator compositions are a composite material comprising a resin matrixwith a variety of filler materials to reduce the overall density orprovide other physical properties.

Certain ablative compositions have included a variety of otherconstituent elements such as metal fillers, colloidal clay fillers,boron and oxygen compounds, polyurethane resins, a mixture of both epoxyand polysulfide resins, and many others too numerous to detail herein.The known art compositions, however, include numerous fillers to achievea desired set of properties such as thermal, mechanical, and others. Asa result, such compositions may be costly and difficult to fabricatewith a relatively large number and variety of fillers. In addition, manyknown art ablator compositions demonstrate relatively low thermal andabrasion resistance performance under high heat flux and pressure loads,for example, as observed in Mach 6 to 8 vehicles.

Ablators with high shear, high heat flux capabilities generally havehigh densities. Lightweight thermal protection systems cannot withstandhigh shear, high heat flux environments. Accordingly, there remains aneed in the art for an ablator composition that is reduced in weightfrom known ablative solutions that also improves thermal performance.Such an ablator would be low in density yet high in abrasion resistanceand durability before, during, and after high thermal loads. Thepreferred ablator would also be relatively low cost and simple tofabricate.

BRIEF DESCRIPTION

In one aspect, a method of forming a thermal protection system for highspeed aircraft is provided. The method includes mechanically working anuncured ablator material into a first surface of a felt layer such thatthe ablator material penetrates a distance into a thickness of the feltlayer thereby forming an region that has a mixture of felt and ablatormaterial, adding additional uncured ablator material to the workedablator material, and curing the combined ablator material.

In another aspect, a thermal protection system for high speed aircraftis provided. The thermal protection system includes a felt layercomprising a first side, a second side, and a thickness separating thesides, the first side operable for attachment to a structure of theaircraft using an adhesive, and a layer of ablator material, a portionof the ablator material mechanically worked into the thickness of thefelt layer from the second side prior to the curing of the ablatormaterial.

The features, functions, and advantages that have been discussed can beachieved independently in various embodiments of the present inventionor may be combined in yet other embodiments further details of which canbe seen with reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an aerospace vehicle capable oftraveling at high speed (greater than Mach 1) through the atmosphere ofthe Earth.

FIG. 2 is a cross-sectional view of an ablator composition formed on astructure of the aerospace vehicle of FIG. 1.

FIG. 3 is an illustration of the components of the ablator compositionof FIG. 2.

FIG. 4 is a flowchart describing a process for assembling a thermalprotection system that contains the ablator composition shown in FIG. 3.

DETAILED DESCRIPTION

The disclosed embodiments are directed to application of an ablatormaterial, for example, Boeing lightweight ablator (BLA), to a needledfelt of Nomex fibers, creating a dual component ablative thermalprotection system. Nomex is a registered trademark of E. I. du Pont deNemours and Company. The described configuration reduces structuretemperature and thermal protection system weight compared to BLA onlyapplications.

Referring to the drawings, the ablator composition of the presentinvention is applied to the exterior surfaces of an aerospace vehicle asillustrated and generally indicated by reference numeral 10 in FIG. 1.The aerospace vehicle 10 is shown flying through the atmosphere of theEarth, where high acceleration and velocities create extremely elevatedthermal loads across the exterior surface, or aerosurface, of thevehicle 10. Accordingly, the ablator composition 20, shown in FIG. 2,provides thermal protection for the vehicle 10 during these extremethermal conditions.

Referring specifically to FIG. 2, the ablator composition 20 is shownapplied to a portion of the outer mold line (OML) 24 of the aerospacevehicle 10. In addition to protecting the OML 24, the ablatorcomposition 20 further provides protection to substructure 30 adjacentthe OML 24. Accordingly, additional structure and/or systems withinclose proximity of the OML 24 are protected from the extreme thermalenvironment by the ablator composition 20.

The application of the ablator composition 20 to an aerospace vehicle 10as described herein should not be construed as limiting, rather thisapplication is merely illustrative of one structure and one operatingenvironment in which the described embodiments have utility. The ablatorcomposition 20 described herein can further be employed with a widevariety of structures and systems that must withstand high thermal loadsfor an extended duration.

Now referring to FIG. 3, specific layers of the ablator composition 20are shown and referenced to a structure 50, for example, the outermoldline 24 of vehicle 10. The layers include an adhesive layer 60 thatis applied directly to the structure 50, a felt layer 62 that is appliedto the adhesive 50, and a Boeing lightweight ablator (BLA) layer 64. Inone embodiment, the felt layer 62 is felt reusable surface insulation(FRSI) that is heat treated. In one specific embodiment, the felt layer62 of FRSI is about 0.160 inches in thickness.

In one embodiment, a process of fabricating the ablator composition 20has been modified to improve the adherence between the felt layer 62 andthe BLA layer 64, specifically to get an additional amount of the BLAlayer 64 absorbed into the felt layer 62, thereby increasing themechanical lock between the two layers.

FIG. 4 is a flowchart 100 illustrating the process of fabricating theablator composition. Specifically, the adhesive layer 60 is applied 102to the structure 50, for example, an aluminum panel. The felt layer 62is then laid 104 onto the adhesive layer 60. In one embodiment, pressureis applied to the felt layer 62 to improve adhesion between the adhesivelayer 60 and the felt layer 62. The adhesive layer 60-felt layer 62interface is cured 106 at room temperature. After curing, a first layerof uncured BLA 64 is applied 108 to the felt layer 62. The uncured BLA64 is then worked 110 into the felt layer 62 as further described below.After the working 110 step, a second layer of uncured BLA is applied 112to the felt layer 62-BLA 64 combination. As only a portion of the firstBLA layer is worked in the felt layer 62, and since the second layer ofBLA 64 is applied 112 before the first layer of BLA 64 is cured, theseare shown as a single layer of BLA in FIG. 3. After application 112 ofthe second layer of BLA 64, the composition 20 is cured as furtherdetailed in the following paragraphs. Application of the BLA, in oneembodiment, is accomplished using a spraying process.

Working 110 the BLA-felt interface, in one embodiment, includes movingthe nomex fibers of the felt layer 62 in several directions and causingrotation of these felt fibers so the BLA is, or can be, worked into thefelt. In one specific application, the felt layer 62 is needled, forexample using a spreading tool, so that the nomex fibers present a moreporous surface to the BLA. During the application of the BLA to the feltlayer 62, a needle or other needle like device may be utilized as adepth gauge to check thickness. Specifically, by stabbing the needlethrough the uncured BLA and felt layer 62 until it bottoms out on thesubstrate, a thickness of the combination may be determined. In analternative embodiment, the felt layer 62 is needled prior toapplication of the BLA to the felt layer 62. In this alternativeembodiment, the manufacturer of the felt layer 62 moves the nomex fibersof the felt layer 62 in several directions as described above.Subsequently, during application of the BLA, the needle is utilized as adepth gauge to check thickness as described above.

In one specific application, the BLA is worked into the felt layer 62 toa depth of about 0.03 to about 0.04 inches. In one embodiment, the BLAis thinned, for example using a solvent, prior to the mechanical workingof the ablator material (BLA).

The curing of the BLA layer 64 includes curing at an ambient temperaturefor up to about 24 hours followed by an oven curing step. In oneparticular application, the oven curing process includes raising thetemperature, from ambient to about 180 degrees Fahrenheit, at a rate ofabout 10 degrees per hour. The ablator composition is maintained at theincreased curing temperature for up to 48 hours. The increased curingtemperature causes a high temperature bond line to be formed between thefelt and the BLA. This bond line is sometimes referred to as amechanical lock between the BLA and the felt fibers. At least one resultof the above described processes is that after the BLA has hardened,gets charred through use (producing silica), there is still a mechanicallock between the felt and the BLA.

In the above described process, the felt of the felt layer 62 trapssolvent of the BLA. The trapping of solvent by the felt layer 62 is anundesired by product of the described processes. This trapped solventtends to cause problems in curing of the felt layer/BLA interfacepredominantly by not allowing the BLA to fully cure. The development ofthe above described curing method has been added to the fabrication ofthe ablative composition to produce a successful bond between the BLAand the felt layer 62. If the BLA is not fully cured, the thermalprotection system will have underperforming mechanical properties.Therefore, the solvent has to be released from the thermal protectionsystem. However, if the solvent is driven off too quickly it can cause aseparation in the BLA/felt layer bondline due to the volatiles expandingfaster than can be permeated through the BLA, which results in blisterswithin the uncured BLA. The specific slow cure process described in thepreceding paragraph, with the slowly escalating temperature allows thesolvent to be slowly flashed out of the BLA, without causingdelaminations

The thickness of the felt layer 62 and the thickness of the BLA 64 areselected based on the particular application. In one embodiment, the BLAis a silicone ablator, filled with a commercial filler, and is describedin U.S. Pat. No. 6,627,697, the disclosure of which is incorporated byreference. The described structure of ablator material worked into feltand subsequent curing provides an ablator that exhibits high shear andhigh heat flux capabilities, and the use of the felt layer helps reduceweight of the resulting dual component ablative thermal protectionsystem as compared to BLA only ablator systems.

Referring back to FIG. 3, the combination of the adhesive layer 60, thefelt layer 62, and the BLA layer 64 are generally formed on a structure50, as described herein. In practice, such a structure is generally apanel of an air vehicle. Upon curing of the ablator material, the panelmay be attached to a vehicle frame. Before or after attachment, thecured ablative material may be milled to a desired thickness, forexample, as required by the application.

The described embodiments illustrate the combination of the high shearand high heat flux capabilities of BLA with the low aerial weight andlow thermal conductivity characteristics of a needled felt layer, forexample, of nomex fibers. The manufacturing method creates a mechanicalbond between the BLA and the felt that can withstand a higher usetemperature than secondarily bonded BLA. As such, the embodimentscombine a low thermal conductivity and low weight thermal protectionsystem component with a high heat flux and high shear capable ablator.The improvement over the prior art occurs because the addition of theneedled felt lowers the weight of the thermal protection system whilemaintaining the high heat flux and high shear capabilities.

Summarizing the described embodiments, the needled felt layer, forexample, felt reusable surface insulation (FRSI) is bonded to thecarrier structure with appropriate adhesive. In one specificapplication, RTV 560 is the preferred adhesive. BLA, thinned with OS-10or other thinning fluid to aid in application, is manually pressed andworked into the felt surface fibers until a continuous layer of BLA ispresent in the felt. Before the BLA in the felt fibers is allowed toflash or cure, a second layer of BLA is then spray applied to the feltto achieve the desired overall thermal protection system thickness. Thematerial is cured via room temperature and/or a heat cycle. The BLAsurface is then machined to a desired outer mold line profile.

The addition of the needled felt of nomex fibers lowers an overallthermal conductivity of the described thermal protection system, whichreduces backface temperature capabilities of the thermal protectionsystem. Further, the felt is a lower weight material than BLA and itsaddition lowers the weight of the system when compared to BLA onlythermal protection systems. The application technique, consisting ofpressing the BLA into the felt surface to create a layer of BLA imbeddedin the felt, provides a mechanical bond between the BLA and felt thatwhen cured that can withstand a higher temperature than an adhesive bondbetween BLA and a structure. Simply, the BLA may get more brittle overtime and the felt provides a type of strain relief function for the BLA.More specifically, the BLA will get more brittle during the dynamicheating conditions associated with flight. For example, the BLA becomesless elastic and more ceramic like when temperatures start to exceed 800degrees Fahrenheit. The BLA characteristics do not change over time atroom temperatures or at temperatures less than 500 degrees Fahrenheit.

The described embodiments provide a high heat flux, low density ablatorcapable of operating in a high shear environment. The use of a dualcomponent thermal protection system uses the optimal performancecharacteristics of two different materials (felt and an ablator materialsuch as BLA) to provide a dual component material with the best possibleperformance. The combination saves weight and decreases outer mold linethickness while maintaining a lower structure temperature, allowinghypersonic vehicles to fly faster and further since weight requirementsare easier to meet and overall drag requirements are reduced.

The above described embodiments are especially suited for use upon asurface of a high speed air vehicle that is commonly referred to as anacreage, which is roughly defined as a top of such a vehicle. However,applications on other vehicles and vehicle locations are contemplated.

This written description uses examples to disclose various embodiments,which include the best mode, to enable any person skilled in the art topractice those embodiments, including making and using any devices orsystems and performing any incorporated methods. The patentable scope isdefined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral languages of the claims.

What is claimed is:
 1. A method of forming a thermal protection systemfor high speed aircraft comprising: mechanically working a first amountof uncured, silicone-based ablator material into a first surface of afelt layer such that the ablator material penetrates a distance into athickness of the felt layer thereby forming a region in the felt layerexposed at the first surface that has a mixture of felt and ablatormaterial; adding a second amount of uncured, silicone-based ablatormaterial to the first surface across the worked ablator material at theregion formed in the felt layer such that the first and second amountsof uncured, silicone-based ablator material are combined; and curing thecombined ablator material.
 2. A method according to claim 1 whereinmechanically working a first amount of uncured, silicone-based ablatormaterial comprises at least one of translating and rotating fibers ofthe felt layer such that the first amount of uncured, silicone-basedablator material is mechanically worked into the first surface of thefelt layer.
 3. A method according to claim 1, wherein mechanicallyworking a first amount of uncured, silicone-based ablator materialcomprises spreading meta-aramid fibers of the felt layer to mechanicallywork the first amount of uncured, silicone-based ablator material intothe first surface of the felt layer.
 4. A method according to claim 1further comprising thinning the ablator material prior to the mechanicalworking of the ablator material.
 5. A method according to claim 1wherein curing the combined ablator material comprises: curing thecombined ablator material at an ambient temperature for up to 24 hours;subsequently increasing the temperature at about 10 degrees Fahrenheitper hour until the temperature is about 180 degrees Fahrenheit; andmaintaining the approximate 180 degrees Fahrenheit temperature for up to48 hours.
 6. A method according to claim 1 further comprising attachingthe felt layer to a structure.
 7. A method according to claim 1 furthercomprising milling the cured ablator material to a desired thickness. 8.A method according to claim 1 further comprising checking thickness ofthe ablator material and felt layer combination using a needle likedevice as a depth gauge.